Regarding the above-mentioned type hydraulic circuit for a working machine, there is known a technique oriented for a hydraulic excavator in which an arm hydraulic cylinder and a swing hydraulic motor are connected in parallel to one hydraulic pump, and a hydraulic fluid drained from the arm hydraulic cylinder is recovered (see, e.g., Patent Reference 1 given below):                Patent Reference 1; PCT Laid-Open Publication WO94/13959        
A hydraulic recovery system provided in that related art includes, in a line via which a reservoir-side line connecting a reservoir and a reservoir port of an arm directional control valve for controlling a flow of the hydraulic fluid supplied to the arm cylinder and a pump-side line connecting a pump port of the arm directional control valve and a hydraulic pump are communicated with each other, a check valve allowing the hydraulic fluid to flow from the reservoir-side line into the pump-side line when the pressure in the reservoir-side line is higher than that in the pump-side line, and it also includes a variable throttle valve disposed in the reservoir-side line. The hydraulic recovery system further includes a pressure sensor for detecting the delivery pressure of the hydraulic pump, a control unit for receiving a pressure signal from the pressure sensor and outputting a drive signal corresponding to the received pressure signal, and a pressure reducing valve for reducing a primary pilot pressure from a pilot pump in accordance with the drive signal from the control unit and producing a secondary pilot pressure as a control signal for the variable throttle valve.
In the related art thus constructed, when the loads acting on the swing motor and the arm cylinder are small and the pump delivery pressure is low, the control unit outputs the drive signal to the pressure reducing valve so as to provide a higher pilot pressure, whereupon the opening area of the variable throttle valve is reduced under the higher pilot pressure and the reservoir-side line is brought into a throttled state. Therefore, the hydraulic fluid drained from the arm cylinder is throttled by the variable throttle valve so that the pressure in the reservoir-side line rises. As a result, a larger part of the hydraulic fluid drained from the arm cylinder flows, as a recovered flow, into the pump-side line through the check valve and joins with the hydraulic fluid delivered from the pump, followed by being supplied again to the arm cylinder. On the other hand, when the load of the arm cylinder or the swing motor increases and the pump delivery pressure rises, the control unit outputs the drive signal to the pressure reducing valve so as to provide a lower pilot pressure, whereupon the opening area of the variable throttle valve is increased. Hence, the pressure in the reservoir-side line becomes substantially equal to the reservoir pressure and the recovery flow rate becomes substantially zero. However, because the pressure on the drain side of the arm cylinder is low, a thrust for the arm cylinder can be ensured.
Thus, with the related art described above, when the loads acting on the swing motor and the arm cylinder are small and the pump delivery pressure is low, the recovery flow rate increases, whereby the speed of the arm cylinder can be increased.